AN
E L E M E N T A R Y T E X T - B O O K
OF
T H E M I C R O S C O P E;
INCLUDING A DESCRIPTION OF THE METHODS OF
PREPARING AND MOUNTING OBJECTS, ETC.

BY
J. W. GRIFFITH, M.D., F.L.S., ETC.,
MEMBER OF THE ROYAL COLLEGE OF PHYSICIANS; CONJOINT AUTHOR OF
THE MICROGRAPHIC DICTIONARY, ETC.
WITH TWELVE COLOURED PLATES,
CONTAINING 451 FIGURES.

LONDON:
JOHN VAN VOORST, PATERNOSTER ROW.
MDCCCLXIV.
[The right of translation is reserved.]

PREFACE.

THE object of this little work is to furnish an elementary course of instruction in the use of the Microscope, and on its application to the examination of the structure of plants and animals. Assuming that the reader has had no previous acquaintance with the Microscope, or with the study of natural history, I have attempted to render the descriptions of the objects as simple as possible. At the same time, the technical terms have been added and explained, in order gradually to render them familiar to the reader, and thus facilitate the future study of larger and more detailed works. The objects figured and described comprise the principal structures and more minute forms of both the vegetable and the animal kingdom, those having been selected which are common and readily procurable.

A chapter has been given upon the optical principles on which the action of the instrument depends (which will assist the reader to understand the operation of its constituent parts), including a sketch of the subject of polarized light. The order in which the subjects are treated is scientific, and particular directions have been given for the examination of the objects.

The small size of the work has necessitated the exclusion of figurative descriptions, so that it is adapted rather for a worker than a reader; at the same time, the matter forms a course, and must be taken as a whole for the proper comprehension of the subjects. The technical terms used are referred to in the [Index], so as to furnish to some extent a glossary of terms; and their derivation is given, to facilitate their recollection. The figures, with very few exceptions, are drawn from nature, and are coloured that the objects may be more easily recognized. The magnifying powers under which they have been drawn are denoted by a small number placed beneath each figure: and the particular attention of the reader is requested to this point; otherwise the whole subject will be utterly confused; so much does the appearance of objects vary under different powers.

Directions are given for preparing and mounting objects, implying that the reader will collect specimens for himself, which is to be strongly recommended as the best method of acquiring a practical and useful acquaintance with the objects. These will serve to furnish permanent landmarks in the great ocean of structural forms, will probably recall in after-years pleasant recollections of early excursions in search of the beauties of nature, and, surely, deepen the conviction of the existence of their All-wise Creator.

J. W. G.

CONTENTS.

A
T E X T - B O O K
OF
T H E M I C R O S C O P E.

CHAPTER I.
THE MICROSCOPE.

THE microscope (from μικρὁς, little, and σκοπέω, to see), so called because it enables us to see objects which are too small to be seen with the naked eye, consists of several parts, each of which has its special use. As the proper management of these is of great importance in the successful application of the instrument to minute investigations, we shall commence with the consideration of their names and uses, including those of the more important pieces of accessory apparatus.

Microscope.—The foot of the microscope is that part which supports the instrument upon the table; it is connected above with the stand, of which it is often considered a part. The stand sometimes consists of a single rod or pillar; but in the best microscopes it is composed of two upright plates, between which, at the upper part, the rest of the microscope swings stiffly upon an axle. Arising from this axle, indirectly through the medium of parts which require no special mention, is an arm, to which the body is fixed. The body is moveable up and down by one or two large milled heads, connected with a grooved rod or pinion, which works in the teeth of a rack fixed to the back of the body, or of the arm which supports the body. The large milled heads form the “coarse movement,” as it is called.

On the top of the arm, or on the front and lower part of the body of the microscope, is placed the “fine movement,” consisting of a small milled head, with a fine screw, for moving the body through very small distances.

Next is the “stage,” or flat plate, upon which the objects to be viewed are placed. This is often so arranged that, by turning two milled heads, the object can be moved backwards and forwards, or from side to side; it is then a “moveable stage.”

The eye-piece slides into the upper end of the body; and the object-glass screws into its lower end.

Beneath the stage is the mirror, which reflects the light through the object, the object-glass, and eye-piece to the eye.

Object-glasses.—The object-glasses are the most valuable parts of the instrument. There are generally three or more of them; and, by means of an “adapter,” any object-glass can be made to fit any microscope. Great care is required in their use, especially to avoid scratching the lower surface of the glass, which is sometimes accidentally done by pressing the surface against any hard body, or allowing such a body to fall upon it. When not in use, the object-glasses should either be put away in the brass boxes or covered with a small bell-glass, to prevent their receiving any injury.

The object-glasses possess various magnifying powers, according to the distance at which they require to be placed from the object for distinct vision: this is not, however, absolutely correct, yet may serve as a general expression. Thus we have a 1-inch, ½-inch, ¼-inch object-glass, &c. The object-glasses, for brevity, are often called powers.

As a beginner may at first have some difficulty in distinguishing a high from a low power, it may be remarked that the size of the lower glass is larger the lower the power: but, in the case of the better object-glasses, the focal distance is engraved on the box in which the object-glass is packed when put away.

As the coarse movement raises or depresses the body and object-glass through comparatively large distances, it must be used only with the lower object-glasses, i. e. those of low or little magnifying power, as the 2-inch, 1-inch, or ½-inch, or to bring the object-glass near the focal distance with the higher powers; whereas the fine movement serves to adjust the higher powers, as the ¼-inch, &c.

If the object-glasses should become soiled on the lower face, this should be wiped very gently with an old silk handkerchief or piece of very soft wash-leather, previously shaken to displace dust. The same method will answer to cleanse the upper surface of the eye-piece.

Great care must be taken that a slide which has been warmed in any experiment be not placed near the object-glass until quite cold.

Mirror.—The mirror has sometimes one silvered face only, at others two—one flat, the other concave. The flat surface is used to reflect the light upon the object when the light is too great with the concave surface.

Beneath the stage, in most microscopes, is a circular moveable “diaphragm,” perforated with holes of various sizes, to allow more or less of the light reflected by the mirror to pass through, as may be required.

When opake objects are viewed, the mirror should be turned aside, so as not to reflect any light through the stage.

Eye-pieces.—With all microscopes, two or more eye-pieces are supplied. These possess different magnifying powers, and are lettered or numbered accordingly; the lowest power with the earliest letters of the alphabet, or with the smallest numbers, thus: A, B, C; or 1, 2, 3, &c.

Forceps.—These are fine pincers, for holding minute bodies to be viewed as opake objects. In use, they are inserted by a stem connected with a joint, in an aperture, generally in the stage; and are moveable in all directions.

Live-box.—This is a brass slide, perforated in the middle, to the aperture in which is soldered a short piece of brass tube, closed at the top with a circular plate of thin glass. A rather wider and longer piece of brass tube slides over the former; this is also closed at the top by a thin glass plate, so as to allow of an object being confined or compressed between the two glass plates. It is used for examining living objects in water.

Knife, &c.—For cutting slices or sections of objects, a very sharp knife with a thin back will be found useful; or a razor may be used for the same purpose. And for picking minute objects to pieces, or dissection, fine needles, cut off short with pliers, the blunt ends being thrust into hair-pencil sticks, will be requisite.

A pair of fine surgical forceps will also be required, for taking up minute objects. These should be without teeth, and the spring-action so weak that the points can be very easily approximated.

Dipping-tubes.—For removing minute objects from water, two or three narrow glass tubes, of different lengths, are very useful. These are called “dipping tubes,” and are used thus:—the tube being held upright between the second finger and the thumb, the fore finger is placed at the top of the tube to close it; the tube is then put into the water until the lower end is close to the object, when, on suddenly removing the fore finger, the water will rise in the tube, carrying the object with it. The fore finger is then again applied to the tube, and, as thus held, the water will not run out. The tube is then held over a watch-glass, or a slide, upon which the water and object will fall on removing the fore finger.

A small glass spirit-lamp will be found very useful. The spirit for burning should be methylated alcohol, or wood-naphtha. As these spirits are inflammable, great care should be taken to keep the stock-bottle away from a candle or other flame, when filling the lamp.

Achromatic condenser.—A very important piece of apparatus, when high powers are used, is the achromatic condenser; it is not, however, usually supplied with the cheaper microscopes. It consists of a brass fitting, placed beneath the stage, into which an object-glass is screwed, in an inverted position, i. e. the small end of the object-glass being placed uppermost. It serves to condense the light to a focus upon the object, so as to illuminate it more brightly; and as it can be elevated or depressed by a milled head and rack-work, the object can be viewed by either converging or diverging rays.

Simple microscope.—For examining the larger kinds of objects, and for dissection, a simple microscope is very useful. This consists of a stand, a stage, and an arm supporting a simple lens or combination of lenses, but without the body of the compound microscope (as the ordinary microscope is distinctively called). For most purposes, common plano-convex or doubly convex lenses are sufficient to form the object-glasses of a simple microscope. With the best microscopes, an “erector,” or tube containing a pair of lenses, fitted within the body, renders the compound microscope capable of answering most of the purposes of a simple microscope.

Polariscope.—An expensive but interesting and useful addition to a microscope is a polarizing apparatus, or polariscope. This consists of a Nicol’s prism, or a plate of tourmaline, placed beneath the stage, and another in the body of the microscope or above the eye-piece; both in brass fittings. The former is called the polarizer, and the latter the analyzer.

Rotating disk.—Another most useful piece of apparatus, for moving opake objects whilst under the microscope, in all directions, is Smith and Beck’s “rotating disk.”

Slides.—The slides upon which objects, especially those to be viewed as transparent objects, are to be placed, should be made of crown or plate glass. They are usually 3 inches long, and 1 inch wide; but I prefer them 2½ inches long, and 1 inch wide, simply because they take up less room in a cabinet, and because they do not project beyond the stage on either side. They should not be more than ¹/₂₀th of an inch thick, and as colourless and clear as possible. The edges should be ground or filed, to prevent their scratching the stage.

Covers.—The covers are square pieces of very thin glass, less in breadth than the slides, so as not to reach their margins; and of various thicknesses, the thicker and stronger being used to cover large objects for examination under the lower powers, and the thinner serving to cover very delicate objects requiring the higher powers.

Side condenser.—For illuminating opake objects, a large plano-convex or doubly convex “bull’s-eye” lens, or side condenser, is used; this is fixed to an arm, which slides on a stand, so as to be capable of being raised or lowered to a suitable height. This is placed between the source of light and the stage, and at such a distance from the latter that the light may be brought to a focus upon the object. Sometimes a “Lieberkuhn” or concave silver reflector is used for this purpose.

These are the most important pieces of apparatus required in examining microscopic bodies. But the beginner will do well, if he have the achromatic condenser and the polarizing apparatus, to lay these aside until he has had considerable practice in examining objects simply with the mirror and the lower powers.

General method of observation.—In the ordinary use of the microscope, the object to be examined is laid upon the middle of a slide, which is placed upon the stage. The object is then brought under the centre of the object-glass, the mirror inclined half towards the light and half towards the object, until the object is seen to be illuminated, when, upon looking through the eye-piece and adjusting the coarse and fine movements, the object as it comes into focus will be seen, as it were, drawn upon a white disk, which is called the “field.”

When the object is wet, it cannot be viewed without the application of a cover, because the water evaporates and condenses upon the under surface of the object-glass.

To avoid the danger of injuring the object-glass, or crushing the object by lowering the body and object-glass too much in adjustment to focus, the best plan is to lower the body by means of the coarse movement until the object-glass appears near the object to the eye placed on one side of the stage, and then to apply the eye to the eye-piece, and turn the milled head so as to raise the body and object-glass until the object is brought into focus.

In the examination of an object, it is best to begin with a low power, so as to obtain a view of the general arrangement of its parts, and then to apply the higher eye-pieces and powers, so that the more minute structural details may be observed.

Illumination.—The illumination or proper management of the light in using the microscope is of very great importance. The best light, especially with the low powers, is daylight, particularly that reflected from white clouds; this is least injurious to the eyes. But as daylight cannot always be used for microscopic investigations, and as cloud-light is insufficient with the higher powers, some kind of artificial light must be supplied. That mostly used is the light of a reading oil lamp or of a gas-burner, a candle being quite useless, on account of the flickering of the flame with the slightest draught. A moderator lamp has the defect of too great height; otherwise this would be better than any other oil lamp. A powerful and excellent light for the highest powers is afforded by a short paraffine-oil lamp.

As intently looking at strongly illuminated objects is injurious to the sight, the amount of light allowed to pass the diaphragm should be no more than is agreeable, and sufficient to show the object distinctly.

In using the higher powers, the field is much less bright with the use of the same light than in the case of the lower powers; and difficulty is often found in obtaining sufficient light for the distinct vision of the object. The achromatic condenser is of most important service here; but it is sometimes requisite, even when this is used, to condense the light upon the mirror by a shallow bull’s-eye; or a large common metallic reflector may be used for the same purpose.

The most convenient manner of proceeding in regard to the use of the individual eyes is to apply the left eye to the eye-piece, so that the right eye may be used in finding the stage-movements, or in moving the slide, without removing the eye from the eye-piece. If this arrangement be adopted, the light should be placed towards the left-hand side of the microscope. But the best way to avoid injuring the sight would be to use both eyes for viewing the objects in turn, although most microscopic observers make use of one eye only for this purpose.

The structure of many transparent objects can be best seen when the mirror is turned more or less obliquely to one side, so as to view them by oblique light, as it is called: we shall refer to this point hereafter.

As a rule, objects are best seen by transmitted light, or as transparent objects, although it is well to examine objects under both kinds of illumination, i. e. by transmitted and reflected light.

If during the use of the microscope, after removing the eye from the instrument, the impression of the light remains perceptible to the sight, the light used has been too strong, or its action too long continued; and the instrument should be at once laid aside for a time.

CHAPTER II.
THE MOUNTING OF OBJECTS.

THE mounting or “putting up” of microscopic objects signifies their preparation in such way that they may be preserved for future reference and observation.

As a general rule, objects should be mounted in that manner by which their structure is best and most clearly shown; but in certain instances the objects are mounted so as to make their structure difficult of detection, that they may form test-objects of the power and quality of the microscope.

Some objects require to be mounted in the dry state, while others are best mounted in liquid; some again as opake, others as transparent objects: these must be considered separately.

Dry opake objects were formerly mounted by gumming them upon small coin-shaped pieces or disks of cork, blackened upon the surface with a mixture of fine lamp-black and thin warm size, laid on with a hair-pencil. They were kept in a drawer, to the bottom of which a sheet of cork was glued, the disk being transfixed by the pin, so that the free or projecting pointed end of the pin could be thrust into the sheet-cork. This plan may still be adopted in the case of common objects, as seeds, &c.; but it is objectionable, on account of the facility with which the bare objects are knocked off or injured by dust.

Hence dry opake objects are usually mounted in such manner as to be enclosed in a cell, the sides being formed by a ring of glass-tube or cork, or a square piece of leather, cardboard, or paper, with a hole cut or punched out of the middle. The glass rings are best; but as they are expensive, some of the other substances are generally used. The size and thickness of the material from which the rings are made must obviously vary according to the size and depth of the object. The rings are cemented to the middle of ordinary slides; and it is best to keep a number of them ready prepared. The cementing material must vary according to the nature of the ring used. If this consists of glass, Canada-balsam or marine glue is best. In using the former, the ring is gently heated over the flame of the spirit-lamp, and a thin layer of the balsam applied to its upper or under surface, by means of an iron wire with a little balsam on its end; it is next warmed over the spirit-lamp, so that the surface is entirely and evenly coated. A clean slide is then slightly heated, the ring laid upon it, and gentle pressure is used to squeeze out the excess of balsam; and the slide is kept at a gentle heat, until on cooling the balsam becomes so hard as not to be indented with the finger-nail. Marine glue is applied in the same way as the balsam, except that prolonged heat is not required to harden it, for it becomes hard on cooling. The balsam may also be replaced by black japan or asphalte.

The pieces of cork, leather, or paper are best fastened to the slides with solution of shellac or sealing-wax in methylated alcohol, or with white hard varnish.

When the ring or piece has been firmly fixed to the slide by either of the above cementing materials, so as to form the sides of the cell, the bottom is to be covered with a piece of black paper, cut to fit it exactly, and fastened to the surface of the slide with a little gum, or of either of the above varnishes. As soon as this is thoroughly dry, the upper surface of the cell-wall, whether of glass or cork, &c., is thinly covered with varnish, and a clean thin-glass cover laid upon it, and very lightly pressed; the object is then permanently preserved.

The main points to be observed are, that the object and varnish are completely dry, and that the cell is thoroughly closed. If the latter be not the case, more varnish must be applied to any little openings which may have been left; and it is better to apply the varnish in very small quantities at a time, the application being renewed as soon as the previous layer is quite dry.

Dry transparent objects are usually small and delicate; for, unless they are so, their structure cannot be well seen. In mounting these, a square piece of note-paper or tracing-paper, with the centre cut out, may be fastened to a clean slide with a little paste, gum, or shellac varnish. When this is thoroughly dry, the object is placed in the vacant space, a clean dry cover laid on, and the varnish applied by means of a hair-pencil to the edges in very small quantities. This will run in between the under surface of the edges of the cover and the upper surface of the paper, and when dry will cement the two together.

Supposing that the object is so delicate that it cannot be removed from the surface of a slide, if it will not be injured by heat, a good plan is to draw a square or circle around the object with a little black japan, then to heat the slide gradually until the japan is not indented with the finger-nail when cold. A clean slide is then laid upon the ring of japan, the whole again gently warmed, until the varnish is softened, and the cover lightly pressed so as to be in contact all round with the varnish. The slide must then be rapidly cooled, by being laid upon a piece of metal, which prevents the varnish from running in so as to spoil the object.

Many dry transparent objects can be preserved by mounting in Canada balsam. This is the best process for mounting objects in general; but only those can be so preserved which are not injured by drying, and which are not rendered too transparent by the balsam. If the object to be mounted in balsam be small, it is thoroughly dried, and then a drop of oil of turpentine added to it upon a slide; the slide is then gently warmed, which causes the turpentine to evaporate. When this has nearly all evaporated, a drop of balsam is allowed to fall upon the object from the end of a wire held at a distance above the flame of a spirit-lamp. A warmed cover is next laid upon the balsam, and gentle pressure applied until the cover is sufficiently depressed. The slide is then kept at a gentle heat until the balsam is quite hard when cold. The superfluous portions may be removed with the point of a knife, and any residues cleaned off with turpentine or a little benzole on a cloth.

Another way consists in laying a cover upon the dry object on a slide, adding a drop of turpentine, and warming the whole over a spirit-lamp until all air-bubbles are displaced, then continuing the application of the heat until most of the turpentine has evaporated. A drop or more of the balsam may next be applied to the edge of the cover, when it will run in and mix with the turpentine. The whole is then gently heated until the balsam is hard when cold, more balsam being added if necessary, to replace the turpentine which has evaporated.

When the objects are large, they should be pressed as flat as possible without injury between two slides, being retained until dry by enclosure between the prongs of an American clothes-peg; or the slides may be fastened at the ends by sealing-wax. When perfectly dry, the object should be immersed in turpentine, kept in a common gallipot, until all the air-bubbles have been entirely displaced, and the object appears very transparent. It is then removed from the turpentine with forceps, drained, laid upon a slide, and melted balsam dropped upon it until it is quite covered. A clean dry slide is then laid upon its surface, and the two slides gently pressed together, the two slides fixed at the ends by sealing-wax, and the whole allowed to cool and dry. If requisite, more balsam is added to fill up any vacuities. When the balsam has become hard, the excess is cleaned away with a knife and turpentine, and the object is permanently mounted.

If the object should be spoiled by the presence of air-bubbles, the slides and object should be immersed in turpentine or methylated alcohol, until the whole of the balsam is dissolved; the remounting may then be proceeded with as at first. If the slides have been immersed in the alcohol (which is the quickest method), the object must be soaked in turpentine before the balsam is reapplied.

If, after an object has been mounted in balsam, on applying heat, bubbles resembling air-bubbles should be formed, the object must not be considered as spoiled; for these are merely bubbles of the vapour of turpentine, and will disappear spontaneously after a little time.

A quick way of mounting in balsam is to drop the melted balsam at once upon the dried object; but as air-bubbles are very apt to be produced in this way, the beginner had better previously apply the turpentine.

As balsam is very viscid, and adheres firmly to everything with which it comes in contact, some care is required in its use. Young microscopists very generally manage to soil the microscope, tables, chairs, papers, books, and even their clothes with it. It may be easily cleaned off, however, with turpentine or benzole.

Moist objects are best preserved, whenever practicable, in glycerine. There are, however, two important objections to its use: one is, that it makes objects very transparent; the other is, that it often wrinkles and distorts them, by withdrawing their watery contents. Hence only those objects can be preserved in glycerine, which are not too transparent, and which are sufficiently firm to resist the tendency to collapse.

When the objects are tolerably flat, and sufficiently firm to bear the pressure of the cover, they may be mounted by adding a small quantity of glycerine to them lying on a slide; the cover is then applied, and a little of the cement mentioned below applied warm with a hair-pencil around the edges of the cover to fasten it to the slide. Care is required that the glycerine applied be no more than sufficient; for wherever it has touched the cover or the slide, the cement will not adhere. Superfluous portions may be sucked up with a piece of clean moist sponge or a corner of blotting-paper.

When it is required to mount a large number of objects in a short time, the cement need only be applied to two opposite sides of the cover, leaving the other two sides open.

When the objects require to be protected from the pressure of the cover, the sides of a cell must be made with the cement or black japan upon the slide before the cover is applied, a further quantity being used to close the cell as usual.

A very strong solution of chloride of calcium may be used for the same purposes and in the same way as the glycerine. It has the advantage of not making the object so transparent; but it has the disadvantage of crystallizing slightly in a dry atmosphere. In most cases, I prefer it to glycerine.

A large number of interesting objects cannot, however, be preserved in either glycerine or chloride of calcium, without their value being impaired by the cause mentioned above. Many kinds of liquid have been recommended for preserving these, all agreeing mainly in being inefficient. The objection to them is, that they are evaporable; and after the object has been mounted for some time, the liquid creeps between the cement and the slide or cover, at some spot, and evaporates; and if the cement be not quite hard, the inner and more liquid portion of it runs into the cell, and spoils the object. A solution, containing a grain of salt and a grain of alum to the ounce of distilled water, is as good as any other; or simply distilled water in which a piece of camphor has been kept. In use, the cell is first formed by making a circle or outline square on the slide with black japan, and heating this carefully until it becomes solid when cold. The object is then laid in the cell, the liquid added, and the cover applied, any excess being removed with blotting-paper. The cell is to be closed with old black japan or gold-size, applied round the margins of the cover with a hair pencil. A second and a third layer of the varnish may be applied upon the first, when it has become hard outside. Although black japan and gold-size are generally used for the cement, I prefer that mentioned below.

When large preparations are mounted in liquid, the cell-walls are either formed of glass rings, or they are built up with four oblong pieces of glass, cemented to the slide and to each other with marine glue. A very good preservative liquid for large specimens is a solution of chloride of zinc, in the proportion of 20 grains to the ounce of distilled water. A mixture of spirit of wine and water, in the proportion of 1 part to 2, or 1 to 4, is often used for the same purpose.

When preparations are mounted, the cement and the adjacent parts of the slide and cover should be coated with a solution of sealing-wax in spirit, which hardens the exterior of the cement.

The cement above alluded to is made by melting together 5 parts of rosin, 2 parts of balsam, 1 part of bees’-wax, and 1 of red ochre. The cement is best kept in a little metallic cup, and melted over a spirit-lamp when used. It should be applied while hot, with a hair pencil; and cools very quickly.

The preservative liquids should be kept in corked bottles, a hair pencil being fixed into the under part of the cork; or, what is better, in stoppered bottles, the stopper being prolonged to a point nearly reaching the bottom of the bottle.

As soon as the preparations are mounted, they should be labelled, the labels being kept ready gummed. The balsam should be kept in a capped bottle, such as is used for holding solutions of gum, with an iron wire for removing portions as required. By keeping, the balsam becomes thicker; it may be thinned by the addition of oil of turpentine, and the application of a gentle heat.

The black japan, &c., may be procured at any oil-shop; the glass rings, cell-sides, covers, &c., from Mr. Norman, 178 City Road, or of the microscope-makers.

Mounted objects should be kept in shallow drawers, and be laid flat—not standing on edge.

Magnifying power.—Before entering upon the consideration of the objects themselves, a word or two must be said upon the magnifying powers. In the plates of this work, the serial number of each figure is expressed by large numerals placed above the objects, while the number of times the object is magnified is indicated by small numerals placed beneath. The latter must be understood to express the number of times the drawing is larger than the object in one dimension. Thus, considering fig. 13, Plate I. to be an inch in width (for it is really somewhat less), being magnified 150 times in the direction of the width, the object itself is about ¹/₁₅₀th of an inch in size; and it is represented magnified 150 times linear, or 150 diameters, as it is called.

A knowledge of the number of times the object is magnified is of the greatest importance in making use of the drawings; for, without it, the observer will be unable to apply such a magnifying power of the microscope as will enable him to see the structural appearances figured in the drawings.

The observer must also be acquainted with the magnifying powers of his microscope with the various object-glasses and eye-pieces. These are usually given when the instrument is purchased. Or they may be determined approximatively thus:—An ivory scale, with ¹/₁₀₀th of an inch engraved upon it, is placed on the stage, and viewed as an opake object, both eyes being kept open; and the size of the image of one of the gradations is measured with compasses, upon the stage as seen with that eye which is not applied to the eye-piece. The number of ¹/₁₀₀ths of an inch contained in the measure obtained with the compasses represents the magnifying power. Thus, supposing the image of the ¹/₁₀₀th of an inch on the scale appears magnified to the length of 1 inch on the stage; the magnifying power is 100 diameters, or 100 times linear. This proceeding is difficult to any one unaccustomed to the use of the microscope, yet by practice it becomes very easy. Other methods, which require the use of the camera lucida, are given in the Micrographic Dictionary.

CHAPTER III.
VEGETABLE ELEMENTS AND TISSUES.

WE may now enter upon the consideration of the microscopic structure of objects, beginning with those which are derived from the vegetable kingdom, as they are more easily procured and prepared for examination than those belonging to the animal kingdom; moreover they are not so transparent, and hence are more readily distinguished under the microscope, which is of importance in the case of an unpractised observer.

Cells.—The elements of which all plants consist are cells. Cells, in their simplest condition, are microscopic, rounded, colourless, closed sacs or vesicles, resembling small bladders (Plate I. fig. 2), and consist of a thin, transparent, colourless, vegetable skin or membrane (a) called the cell-wall. The cells are well seen in a little of the pulp of an apple (fig. 2), or in a section of almost any soft part of a plant. A high power is usually required to show them distinctly, on account of their minute size. The outline of the cells is seen to be double, one line indicating the inner, the other the outer, surface of the cell-wall, the space between the two lines corresponding to the thickness of the cell-wall.

In the pulp of the apple, the cells are loosely connected, and so retain their rounded form; but in most parts of plants, the cells become crowded and squeezed together, from their ordinary or normal expansion being limited in certain directions, so as mutually to alter each other’s shapes. The sides then lose their originally rounded form and outline, becoming more or less straight ([Pl. I.] figs. 1 & 4),—the cells at the same time mostly adhering to each other, so as to be separated with difficulty.

The forms thus produced are various and interesting, and have all received names by which they are distinguished. They are described in works on botany in two ways—according to the outline (which is the most common, as this expresses the appearance usually presented in sections and on the surfaces of vegetable structures), or according to the entire or solid form, which it is often a difficult matter to determine.

Cellular tissue.—Cells aggregated thus form a tissue, which is called cellular tissue or paren´chyma (παρά, among, and ἕγχυμα, poured substance), because it fills up the interstices of the other tissues of plants.

In technical descriptions, the cell-structure is often left out of consideration; and bodies composed of parenchymatous tissue are described as being reticulated or netted, because the united sides of the cell-walls appear as a network covering the surface.

It must be understood that parenchymatous cells are such only as have the three dimensions of solidity (viz. the length, breadth, and depth) nearly equal.

Intercellular passages.—The observer will not have examined many sections of cellular tissue, without noticing certain irregular black lines running between the cells, as in a piece of a Geranium-(Pelargo´nium-) leaf ([Pl. I.] fig. 1). These lines arise from the existence of passages between the cells, containing air; and they are called intercellular passages. By gently warming a section containing them in water over a spirit-lamp, or by moistening the section with a drop of spirit, the passages will be filled up with the liquid, so as to become transparent. When the intervals between the cells are larger and broader, they are called intercellular spaces.

So far, cells have been considered simply in regard to their form, as vesicles, either rounded or altered in shape by mutual pressure. We have now to notice the matters contained within the cells, or the cell-contents.

Cell-contents.—In most cells, especially when young, a minute, rounded, colourless body may be seen, either in the middle or on one side, called the nucleus; this is very distinct in a cell of the pulp of an apple ([Pl. I.] fig. 2 b). And within this nucleus is often to be seen another smaller body, frequently appearing as a mere dot, called the nucle´olus.

The nucleus is imbedded in a soft substance, which fills up the entire cell ([Pl. I.] fig. 2 c); this is the pro´toplasm (πρῶτς, first, πλἁσμα, formative substance). As it is very transparent, it is readily overlooked; but it may usually be shown distinctly by adding a little glycerine to the edge of the cover with a glass rod, when it contracts and separates from the cell-walls, as in the lower cell of fig. 2. The protoplasm in some cells is semisolid and of uniform consistence, while in others it is liquid in the centre, the outer portion being somewhat firmer and immediately in contact with the cell-wall. In the latter case, it forms an inner cell to the cell-wall, and is called the primordial utricle. The terms “protoplasm” and “primordial utricle” are, however, used by some authors synonymously.

The protoplasm is the essential part of the cell, and it forms or secretes the cell-wall upon its outer surface in the process of formation of the cell considered as a whole. It is also of different chemical composition from the cell-wall, being allied in this respect to animal matter.

Chlor´ophyll (χλωρὀς, green; φὐλλον, leaf).—On examining a section of any green part of a plant, as the green substance of a Geranium-(Pelargonium-) leaf, it will be seen that the green colour does not arise from the whole substance being coloured, as appears to be the case to the naked eye, but from the presence of little grains or granules of a green colouring-matter in the protoplasm of the cells. This green matter is called chlorophyll. If the cells be crushed, the granules will escape, and can be examined in the separate state. Chlorophyll is most abundant in those parts of plants which are exposed to the light.

Starch.—In many cells of plants, particularly those which have attained their full growth, other granules, larger than those of chlorophyll, and colourless, are met with; these are the starch-granules ([Pl. I.] fig. 3). They are usually rounded or oblong, and exhibit on the surface a number of rings, one within the other, or concentric, as it is called. In the centre of the innermost ring is a black dot or streak, arising from the presence of a little pit or furrow, and called the hilum.

The starch-grains may be readily seen within cells in a thin section of a potato ([Pl. I.] fig. 4); here they are very numerous, and larger than in most other plants. A separate grain is represented in fig. 3.

The appearance of rings in the separate grains arises from the starch-granules being composed of numerous concentric coats or layers, like those of an onion.

A very simple and striking method of determining whether any granule is composed of starch or not, consists in adding to it, when placed in water on a slide, a drop of solution of iodine. As soon as this touches the granule, it assumes a beautiful purple colour, the depth of tint depending upon the quantity of the iodine-solution; if this be very considerable, the granule appears almost black. The section of potato forms a very interesting object when moistened with the iodine-solution, the starch-granules becoming beautifully coloured, whilst the cell-wall remains colourless, and the protoplasm becomes yellow.

The form of the starch-granules differs in different plants, so that the kind of plant from which starch has been derived may be distinguished by attention to the size, form, and structure of its starch-granules. Thus, the granules represented in [Pl. I.] fig. 3, which it will be noticed are all drawn under the same power, are derived from different plants,—a being those of wheat-flour, in which the hilum is obscure, and the rings faint; b is a granule of West Indian arrowroot, in which the hilum forms a transverse crack; c is a granule of potato-starch, in which the hilum is a dot, and the rings are very distinct; d represents the compound granules of the oat, the separate granules being figured below; e is a granule of lentil-starch, with its long dark hilum and elegant oval concentric rings; and f represents a compound and separate granule of rice-starch. It will be noticed that the granules of oat-and rice-starch are angular, as it is called.

The knowledge of the peculiar forms of the starch-granules is important in a practical point of view, for it enables us to recognize them when mixed as an adulteration with other substances, and also to distinguish the different kinds of starch from each other. Thus table-mustard, as it is called, is principally composed of the cheaper wheat-or pea-flour, which is easily recognized by the structure of the starch-grains. Arrowroot is considerably dearer than potato-starch; hence in trade the latter is fraudulently sold for the former, the adulteration being detected with difficulty by the eye, but easily under the microscope. Again, rice is largely mixed with wheat-flour, as it makes inferior flour into very white bread; and this may also be readily detected under the microscope. The reader can now understand how valuable the microscope is in detecting adulterations, with a knowledge of the various forms and structures of substances, especially with the aid of a few chemical tests.

Starch-grains are altered by boiling in water, becoming swollen and often changed into curious forms, the rings becoming faint or disappearing. If a piece of boiled potato be examined, the starch-granules will seem to have vanished from the cells, which are swollen and covered with an irregular kind of network. The network consists of parts of the protoplasm situated in the interstices of the starch-granules, and solidified or coagulated by the heat. On crushing the cells by pressing upon the cover, the starch-granules will escape, swollen and partly fused together; but they may easily be recognized as consisting of starch by the iodine test.

The granules of “tous les mois” starch are particularly well adapted for showing the concentric rings, the granules being about twice as large as those of the potato.

Starch-granules are best examined in water; and a small quantity only of the starch must be placed on the slide, if the structure of the granules is to be seen clearly. They may be mounted in glycerine, although this makes them very transparent.

To those who possess a polariscope, starch-granules are particularly interesting, as they exhibit a black cross, and, with a plate of selenite laid beneath the slide, a beautiful play of colours.

In addition to the starch and chlorophyll, the cells of plants contain other matters, as gum, sugar, &c.; but as they are dissolved in the cell-liquid, they are not visible. In the cells of certain plants, however, spherical globules, with light centres and black outlines, will be met with: these consist of oil.

Raph´ides.—Lastly, occurring in the cells of plants, especially such as are soft and juicy (succulent), will be found minute, hard, colourless crystals, called raphides (ῥαφὶς, a needle). These are most frequently needle-like or acicular (acus, a needle), but sometimes prismatic or rod-like with flat sides; they are also not unfrequently grouped into little tufts. They may be readily found in a piece of the stem of garden-rhubarb ([Pl. I.] fig. 5 a), or of the common balsam.

Porous and spiral cells.—The walls of the cells of cellular tissue are sometimes covered with little dots ([Pl. I.] fig. 11 a), or slit-like markings; the cells are then called porous cells. A specimen of them may be obtained from a section of the pith of the elder (Sambúcus nígra).

Sometimes cells exhibit the appearance of a spiral line marking their walls, as if a little bell-spring were coiled up in them ([Pl. III.] fig. 2 a). These are called spiral cells, or spiral fibrous cells, and the tissue formed by them is called fibro-cellular tissue.

We now leave the cells of ordinary cellular tissue, to examine those in which the dimension of length predominates, so that they form tubular cells; and first of those required to possess strength and firmness, combined with flexibility. These qualities are met with in the cells constituting

Woody tissue.—Of this there are two forms, called respectively wood-cells and woody fibres.

The wood-cells are moderately long, more or less tapering and overlapping at the ends; and the cell-walls are thickened, so as to possess considerable firmness. These cells are found in the wood of stems, as in the white woody portion of an ash stick, that of a lime-tree, the stem of a Chrysanthemum, &c. ([Pl. I.] fig. 6). They are closely packed, and the tissue formed by their union is called prosen´chyma (πρὸς, close, ἔλχυμα, tissue).

In the other kind of woody tissue the cells are very long and slender, strong, yet flexible, gradually tapering at the ends, where they overlap each other; and they have thick walls, so that, when divided transversely, the cavity appears almost filled up ([Pl. I.] figs. 5 d, 9, & 7 b). This tissue is called woody fibre or pleuren´chyma (πλευρἀ, rib, ἔγχυμα), from its strength.

The walls of the cells of woody tissue are often covered with dots, either simple or with an inner dot (Pl. I. fig. 6 b, fig. 11 b), or with streaks ([Pl. I.] fig. 6 a) or with a spiral fibre (fig. 11 b, c), either alone or with dots also.

This tissue is of great importance in plants, from its strength and flexibility; it forms a considerable part of the veins of leaves, the inner bark (liber), and of the wood of the stems of trees. It is also very useful to man: for it constitutes hemp, of which rope and string are made; flax, of which linen is made; cocoanut fibre; bast, used by gardeners for tying up plants, which is the inner bark of the lime; and jute, which is the inner bark of an Indian lime-tree.

In the white woody part of the stems of trees belonging to the fir-order (Conif´eræ), as a piece of deal or pine, which is mainly composed of wood-(prosenchymatous) cells, the cells exhibit rows of minute circular markings ([Pl. I.] fig. 10). These were formerly supposed to be solid bodies or glands; hence the tissue is still sometimes called glandular. Within the outer ring of each marking is an inner central dot, or sometimes an oblique streak. The side view of the cells ([Pl. I.] fig. 8 a), which is seen in a tangential section, shows that the markings are minute pits, each being opposite to one of an adjacent cell, and sunk inwards towards the centre of the cell, the inner dot or streak being a thinner portion of the cell-wall. This glandular tissue of the Coniferæ is interesting as forming a test-object for the defining power of the microscope, which should show the two rings sharply and free from colour; the section of the wood should be examined as a dry transparent object.

The difference between the woody fibre and the wood-cells of coniferous wood may also be seen well in a piece of deal, as cut up for fire-wood. If the end of a stick of this be examined with the naked eye, parts of brown rings will be seen traversing the whiter portion of the wood. These brown rings consist of woody fibre; the white portion of wood-cells. On making a very thin transverse section, the interior of the woody fibres is seen to be almost entirely filled up ([Pl. I.] fig. 7 b), while the cavity of the wood-cells is much more open ([Pl. I.] fig. 7 a); the former also contain globules of turpentine.

It must be remarked here that some botanical authors include both forms of woody tissue under the term prosenchyma. But, as we shall see hereafter, the form of the prosenchymatous cells being sometimes used as a character for distinguishing the cells of leaves, to which the term pleurenchymatous cells would be inapplicable, the above distinction will be found important.

Vessels, vascular tissue.—In the next form of tubular cells, these are broader and softer than the cells of woody tissue, thin-walled, and the ends pointed; and their walls exhibit spiral or ring-like markings, or rows of dots ([Pl. I.] fig. 5 c, e, b), indicating the existence of one or more spiral fibres or rings. When the vessels contain spiral fibres, they are called spiral vessels ([Pl. I.] fig. 5 c); when they contain ring-shaped portions of fibre, they are called annular (an´nulus, a ring) vessels ([Pl. I.] fig. 5 e); and when the spaces between the fibres are partly filled up, leaving only dots, the deposit forming a kind of network, we have a reticulated (réte, a net) vessel ([Pl. I.] fig. 5 b). This tissue can easily be obtained from a piece of cooked rhubarb, the stem of a balsam, or from any soft-stemmed plant. Vessels very frequently contain air.

Ducts.—The tubular cells forming ducts ([Pl. I.] figs. 5 b, 11 c) are large, more or less flattened or blunt at the ends (truncated); and the cell-membrane at first closing the ends is often removed or absorbed, so that the ducts communicate with each other, to allow of the free passage of the sap through them. Their walls are invariably covered with markings, consisting of either simple or bordered dots, resembling those met with in the preceding forms of tissue. The ducts are often easily recognizable with the naked eye, in transverse sections of stems, by the large pores which they form in the wood. These may be well seen in a section of a piece of cane. The tissue composed of dotted ducts is called bothren´chyma (βὁθρος, pit); but the term is principally applied to those ducts in which the dots are simple, i. e. have no inner dot.

The structure of the above forms of tissue may be best understood in relation to their development. It has been stated that the essential part of the cell is the protoplasm. As cells grow older, new matter is deposited by the protoplasm upon the inner surface of the cell-wall, either to a small extent, evenly and uniformly, as in ordinary parenchyma, or unevenly, in the form of spiral layers, forming fibres or bands, leaving bare spaces, where the original cell-wall exists alone. The matter thus deposited is called secondary deposit, the original cell-wall being the primary deposit. When the secondary deposit covers the interior of the cells except at certain slit-like spaces, we have the appearance figured in [Pl. I.] fig. 6 a. When the deposit forms a spiral fibre, or a series of rings, we have the spiral or annular vessel or duct. And when the interspaces between the coils of a close spiral fibre are filled up except at certain spots, we have the dotted or reticulated vessel or duct.

In many instances, these deposits are present together: thus, sometimes the outermost deposit leaves rounded pits or dots, while an inner portion forms a spiral fibre ([Pl. I.] fig. 11 b); or one layer leaves simple rounded pits, while the other leaves smaller slits or dots placed opposite the former ([Pl. I.] fig. 6 b).

In some cells the cavity is almost entirely filled up by secondary deposit, which leaves minute canals radiating from a small cavity in the centre to the circumference, as seen in the transverse section of a plum-stone ([Pl. I.] fig. 56); here the canals appear as dark lines. In others, again, the secondary deposit forms several distinct layers, leaving channels very similar to those of the last; an example is met with in the gritty tissue of the pulp of a pear.

The obvious use of the pits and channels in the above tissues is to preserve the permeability of the walls of the elements, which would be destroyed if the walls were equally thickened all over.

Cell-formation.—New cells are formed by the division of old or parent cells. The actual process of division is difficult to observe, as it requires prolonged observation; but cells are often met with in all stages of division, of which some instances will be pointed out hereafter. The cell-division takes place in two ways, either according to the endogenous (ἔνδον, within, γεννἁω, to produce), or the exogenous (ἔξω, outside, γεννἁω) method. The manner in which the division takes place in the former is this:—At first a slight indentation or constriction of the protoplasm occurs at the line of division; this deepens until the protoplasm is completely divided. The freshly divided surfaces then become coated with a new portion of cell-wall, so as to make two or more new cells, which either remain in contact or separate from each other. In some cases, the divided portions of protoplasm become coated all over with new cell-walls.

In the exogenous process, a portion of the protoplasm protrudes from the surface of the cell, carrying the cell-wall before it, so as to form a little bud-like body; this is next cut off at its point of junction with the parent-cell, and coated, as in the first case, with a new cell-wall, so as to form a new cell.

Preparation.—In examining the vegetable elements and tissues, very thin sections must be made with a razor or thin sharp knife; these are then to be placed in a little water on a slide. As the structures are all minute, the distinctness with which they are seen will mainly depend upon the proper thinness of the sections. When sections of dry stems are to be examined, the black margins of the air-bubbles contained in the cells often render the structure indistinct; these must therefore be displaced by first wetting the tissue with methylated alcohol, and then adding water to it in a watch-glass or on a slide; or the tissue may be soaked in warm water for some hours: and this is mostly requisite in preparing thin sections of dry tissues.

Attention must also be paid to the manner in which the section is made, or the direction in which the portion of the plant is cut. There are three important directions which must be distinguished, producing transverse, longitudinal, and tangential sections. If the cuts be made across the length of a stem, for instance, the section is called transverse. If the cuts be made in the direction of the length, through the centre, the section is longitudinal; and if the cuts are made in a direction parallel to a line running down the centre of the stem, but nearer its margin, it is a tangential section. It is scarcely necessary to mention that an oblique section is intermediate between a transverse and a longitudinal section.

CHAPTER IV.
VEGETABLE ORGANS.

THE vegetable elements and tissues which have been described form, either separately or by their combination in various ways, the organs of plants. To these we shall now pass, and consider the structure of the principal organs of the members of the vegetable kingdom.

Leaves.—Leaves in their simplest form consist of a single sheet or layer of parenchymatous cells or cellular tissue, an example of which may be found in almost any moss ([Pl. III.] fig. 30). The granules of chlorophyll will often be very distinctly seen in these cells. The first addition to this form of leaf is a row or two of prosenchymatous cells running longitudinally down the middle of the leaf, so as to form a rudimentary vein or nerve. In other and more highly developed leaves, the layers of cells are numerous, and traversed by bundles of wood-cells, vessels, and ducts (fibro-vascular tissue), forming the veins,—the entire surface being covered with a skin or membrane, called the epidermis.

Epider´mis (ἑπἱ, upon, δἑρμα, skin).—This membrane is composed of one or more layers of colourless, closely packed cells ([Pl. I.] figs. 13 & 28), the colour it occasionally exhibits usually arising from some of the underlying cells of the leaf being seen through it, or remaining adherent to it when stripped from the leaf. It is easily separated, by making a cut in a soft leaf, and peeling it off with a fine pair of forceps, or by soaking a leaf for some time in water and then stripping it off. It must be remarked that the epidermis covers not only the leaves, but every part of the plant.

Hairs.—Arising from the epidermis are the hairs of plants. These are thread-like or filamentous prolongations of the epidermis beyond the surface of the leaf ([Pl. I.] fig. 12), consisting of cells arranged end to end. They are often branched, sometimes star-shaped (stellate) (fig. 28), and present great varieties in form, as shown in the figures, the plants from which these were drawn being mentioned in the Description of the Plates. Sometimes hairs terminate in a little head ([Pl. I.] figs. 12 c, d, e), the cell or cells composing which secrete a colouring or a viscid substance; they are then termed glandular. The hairs of plants are particularly interesting to the microscopic observer, not only on account of their curious forms, but in connexion with the remarkable phenomenon of the circulation of the cell-contents, or rotation, as it is called, observable in them. This is difficult to be perceived by any one unaccustomed to microscopic observation, because the particles by which the motion of the cell-contents becomes evident are exceedingly minute; but practice in the use of a high power will overcome this difficulty. The hairs which exhibit the phenomenon best are those of the American Spiderwort (Tradescan´tia Virgin´ica), which is to be found in every garden. It may, perhaps be recognized thus:—The plant is about a foot and a half high; the leaves are sword-shaped and channelled, and the flowers are purple, in heads, and 1½ inch in diameter. The hairs are attached to the sides of the stamens, towards the lower part or base. The stamens should be carefully picked off with forceps, and placed on a slide in a drop of water; the hairs should then be separated with the mounted needles, and a cover applied. Under a low power, the hairs are seen to be beaded or monil´iform (moníle, a necklace), and of a fine purple colour ([Pl. I.] fig. 22). On applying a high power, as the ¼-inch, the individual cells will come distinctly into view, and the nucleus will be seen very clearly as a roundish granular mass ([Pl. I.] fig. 23 a). On carefully examining the cell-contents, delicate lines will be observed radiating irregularly from the nucleus, some passing to the top of the cells, while others run towards its base, as in the figure; and on very close inspection, the portions of protoplasm of which these lines consist, will be found to move slowly and steadily, the motion becoming perceptible by means of the minute granules of which the protoplasm consists. The currents return at the ends of the cell, there being no passage of the contents of one cell into the cavity of either of those adjacent. During this examination, it will be noticed that the surface of the cell-wall is striated with fine wrinkles.

It may be remarked that the hairs should be taken from flowers which have only just opened; for this curious and inexplicable rotation is connected with the growth of the cell; and when this has attained maturity, it no longer occurs. The phenomenon may be observed in many other hairs of plants, as those of common groundsel (Senécio vulgáris) ([Pl. I.] fig. 12 a, b), and in the cells of the leaves of some water-plants; but I must refer to the article “Rotation” in the Dictionary for further information.

The most important variety of hair is that derived from the Cotton-plant (a kind of Mallow), and forming the cotton of commerce. These hairs spring from the epidermis of the seeds. The cells composing it are very long and soft, becoming flaccid and easily bent when dry ([Pl. IX.] fig. 13).

Stings.—Stinging hairs or stings may be well illustrated by reference to the common large nettle (Urtíca dioíca). In this plant they consist of a thick-walled cell, bulbous at the base, which is imbedded in the epidermis ([Pl. I.] fig. 21), the pointed end being terminated by a very minute dilatation or knob. The sting contains an acrid liquid, which escapes when the little knob is broken off in wounding the skin, and produces the well-known irritation. By the side of the figure of the sting is represented the point of a fine needle (fig. 20), showing that the expression “sharp as a needle” has no force when microscopic bodies are in question.

Stom´ata (στὁμα, mouth).—On viewing a strip of epidermis, the observer will be sure to notice certain oval or roundish bodies ([Pl. I.] fig. 13 a), composed of mostly two kidney-shaped cells in apposition but leaving a chink between them; these are the stomata. They communicate beneath with the intercellular passages, of which they may be considered the mouths; and by their agency a direct communication is established between these passages and the air. The two cells which guard the orifice are termed the “guard cells.”

Stomata are most numerous on the under surface of leaves; they are entirely absent in plants growing under water, and in most of the lower plants. In many of the stomata, viewed in the ordinary way, the air situated between the guard cells is indicated by the black spot or dot present; but after a time, or by the application of a gentle heat to the slide, the air becomes displaced by the water, and their structure becomes very distinct.

In certain plants, the epidermis is imbued with flint or sil´ica; so that even when burnt to an ash the stomata are still quite distinct. Examples of this may be found in the stalk or culm of grasses, as in straw, the shining epidermis of which is siliceous; or the epidermis of canes. Among the lower plants, this peculiarity is especially curious in the species of Equisétum, or mares’-tails.

The manner in which the veins of leaves are arranged is worthy of special attention, as it forms one of the characters by which the two leading divisions of the Vegetable Kingdom are characterized. Thus in one of these divisions the veins are branched, so as to form a network throughout the leaf; the plants with these netted veins, to which belong our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens; while in the second division, the veins run parallel to each other, being little or not at all branched, and not forming a network. The plants with parallel veins, among which are our grasses, lilies, &c., are the Monocotylédons or En´dogens.

Stems.—In the stems of plants, the tissues are arranged round a centre; otherwise, in the simpler and lower plants, they agree in structure with leaves, the centre being occupied by some element of fibro-vascular tissue, as simple wood-cells, a few vessels or ducts.

In the higher or flowering plants, the stem exists in two distinct forms, corresponding to the differences above noticed in the arrangement of the veins of the leaves; these must be considered separately.

In the Dicotyledons or Exogens ([Pl. I.] fig. 36), the centre of the stem, in a transverse section, is seen to be occupied by the pith or medulla, which is represented in the figure by the innermost circle. Immediately outside and around this is a narrow ring, indicating the section of a sheath to the pith, and called the medullary sheath. Next comes a broad ring of wood of the first year’s growth (fig. 36 a), traversed, from the pith to the bark, by wedge-shaped paler rays, termed the medullary rays. Outside the first year’s wood is the newer and paler wood of the second year (b); and so on, a new ring of wood being added outside the preceding layer for each year of growth of the stem.

On the outer side of the wood is the inner bark or liber (fig. 36 c); and outside this is the spongy outer bark (d), covered by its epidermis.

These structures are of different composition, as may be best seen in longitudinal sections. The pith and the medullary rays consist of cellular tissue, the cells being mostly rounded in the former, and more closely pressed together and squarish in the latter. The medullary sheath consists of vascular tissue; and the wood, of wood-cells traversed longitudinally by bundles of vascular tissue and ducts, the latter being larger and more distinct towards its outer boundary. The liber is composed of woody fibre, and the outer bark of cellular tissue.

The new woody matter being deposited outside the old, between the bark and the previously formed layer, gives origin to the term exogen (ἔξω, outside, γεννἁω, to produce). These structures may be examined in the section of a branch of the lime-tree or lilac.

In the Monocotyledons or Endogens ([Pl. I.] fig. 37), there is no distinct bark, nor pith, nor medullary rays—the entire stem consisting of cellular tissue with isolated bundles of fibro-vascular tissue scattered through it. Moreover the new substance is added to the centre of the stem, or within the old; hence the term endogen (ἔνδον, within, γεννἁω). A section of a piece of cane will exhibit this structure.

To examine the structure of stems, sections must be made in various directions. The relative position of the component parts of a stem are best seen in a transverse section; but the structure of the tissues is most evident in longitudinal sections, and under the higher powers. The annual rings of the Exogens are best observed in transversely sawn-off pieces of perfectly dry stems, which have been polished with sandpaper, and varnished with spirit varnish.

Roots.—The structure of roots is very similar to that of stems; there is, however, no distinct pith, nor are there stomata on the epidermis; and the vessels are replaced by ducts. The very fine rootlets or radicles of water-plants often show the rotation of the protoplasm very distinctly.

Flowers.—The various parts of flowers, being each a modified leaf, present the same general structure as the latter. As the reader may not be acquainted with the names of these parts or organs in the higher plants, and as we shall have to compare them with their representatives in the lower forms of vegetable life, it will be well briefly to indicate them. A common and beautiful yet despised flower ([Pl. I.] fig. 32) may serve for illustration; this is chickweed (Stellária média), which can be found everywhere. The outermost circle of flower-leaves, which forms a kind of cup to the rest of the flower (a), is the calyx; the separate leaves being called the sepals. The row within this, in most flowers consisting of brilliantly coloured pieces, forms the corolla (b); the individual pieces being the petals. When the two kinds are equally coloured, or not distinguishable, the whole is called the perianth, as in a tulip. When the segments of the perianth are dry and chaffy, as in the flowers of grasses, the outermost are said to constitute the glumes, and the innermost the paleæ. Within the ring of petals are certain thread-like organs called stamens (c); and these consist of a filament (fig. 39 a), surmounted at the top or apex by the anther (fig. 39 b), which is usually coloured, and consists of two lobes. The anthers when ripe burst, and discharge a coloured dust; this is the pollen. Lastly, within the stamens is the central organ of the flower, the pistil, and sometimes there are several of them. The pistil consists of three parts, viz. a swollen base, the ovary (fig. 41 b), surmounted by a column or style (fig. 41 a), and which is crowned by a viscid and often hairy summit, the stigma (fig. 40*). In chickweed there are 3 styles.

It must be remarked that, in the flowers of some plants, stamens alone are present, while others contain pistils only, although most flowers contain both organs. When the stamens and pistils occur in separate flowers on the same plant, the plant is said to be monœcious (μὁνος, single, οἶκος, family); when all the flowers of distinct plants contain either stamens only or pistils only, the plant is diœcious (δις, twice, οἶκος); and when the stamens and pistils occur together in all the flowers of the same plant, the plant is said to be hermaphrodite. These terms had their origin in the idea that the differences of plants in respect to these organs were analogous to those of the sexes in animals. All the parts of a flower have their special uses: thus the calyx and corolla protect the delicate organs enclosed by them, until they attain maturity. The petals also, by their brilliant colours, attract insects which feed upon or collect the honey of the flowers; these at the same time conveying the pollen which adheres to their bodies from one flower to the stigma of another. The stamens and pistils are organs of fructification, it being essential for the fertilization of the flowers that the pollen should come into contact with the stigma. We will now consider some interesting points of structure in these organs.

Petals.—The petals often form most beautiful microscopic objects, on account of the curious shape and structure of the cells of their epidermis, and the splendid tints of the colouring matters contained in them. As petals are mostly too thick to allow of the cells being distinctly seen in the entire state, a little cut should be made in them while gently stretched on the finger, and the epidermis carefully stripped off with forceps; the strip should then be laid on the slide in water as usual: in this way the curious patterns of the epidermic cells will become very distinct. The petals of a red geranium (Pelargónium) may be used to illustrate them ([Pl. I.] fig. 24). The structure may be best understood by reference to the epidermis of the leaf of a geranium ([Pl. I.] fig. 13), in which the cells present wavy or undulate walls. In the petal (fig. 24), the walls are inflexed at tolerably regular distances, so as to give rise to the appearance of a row of teeth lining the cell. If the strip of petal be folded, so as to exhibit the side view, it will also be seen that the cells project outwards from the surface to form a bluntish point or papilla, or the petals are papillose as it is called; and the surface of the membrane around the papillæ is finely wrinkled, so as to present the appearance of very delicate radiating lines or striæ. Intermediate degrees of this inflexion may be found in various flowers, between the slight condition seen in fig. 13 and the extreme state of fig. 24, as in the snapdragon (Antirrhínum május).

Anthers.—The cavities of the anthers are lined with fibro-cellular tissue, the fibres of which aid in discharging the pollen; this may be seen by dissecting an anther of London pride (Saxif´raga umbrósa), or of a wallflower (Cheiran´thus cheíri) in water. It also exists in chickweed.

Pollen.—The pollen consists of minute grains called the pollen-granules. They may be viewed either in the dry state as opake objects, or when immersed in water as transparent objects. As it is often difficult to moisten them, they may be touched on the slide with a little spirit, and then a drop of water added. Their forms are very varied and curious, but they are difficult of observation from their minute size. They consist of one or more coloured cells, and these cells are remarkable for their surfaces exhibiting spines, networks, folds, and markings of various kinds. Thus in the primrose the pollen-granules are cylindrical, the surface being furrowed ([Pl. I.] fig. 16); in the sunflower the granules are spherical, and covered with tubercles surmounted by spines (fig. 17); in the garden convolvulus the surface of the spherical granules is covered with an elegant network, in the meshes of which are also situated spines (fig. 18); and in the granules of chickweed the surface presents pits, with minute tubercles in the centre (figs. 30 & 31). The pollen-granules are often considerably altered by immersion in water; so that, in judging of their structure when examined in water, the resulting alteration must be taken into account.

When ripe pollen-granules have been immersed in water for a short time, one or more minute tubes will be seen protruding from their surface; these are the pollen-tubes, and the granular protoplasm contained in them is called the fovil´la. In the process of fertilization of the flower, the pollen-granules fall upon the viscid stigma; the pollen-tubes are then protruded, and, passing down the intercellular spaces of the style ([Pl. I.] fig. 14), enter an aperture in the ovule or young seed, which is thus endowed with the power of growing into a new plant. The pollen-tubes are often very long, and they do not exist fully developed in the pollen-granules, but grow down the style, just as the little rootlet of a seed grows into the soil. The style of a crocus will serve for dissecting out with mounted needles the long and very slender pollen-tube ([Pl. I.] fig. 15).

O´vary.—The ovary by its growth and enlargement becomes the fruit. There are many interesting microscopic structures to be found in fruits and the seeds they contain, a few of which may be noticed here.

On examining the surface of the rind or pericarp (περἰ, around, καρπὀς, fruit) of an orange, little dots will be seen, paler than the rest of the surface. These are receptacles of secretion, or glands, containing the evaporable or volatile oil upon which the fragrance of the orange depends. They consist of loose cells, surrounding a central cavity, and are imbedded in the rind.

Other receptacles of secretion, called vittæ (vitta, a band), occur in the wall (pericarp) of the fruit of the Umbelliferæ, or Parsley Order of plants, and their arrangement forms characters for distinguishing the genera. They may be well seen in caraway-seeds; for the caraway-plant is one of the Umbelliferæ. It must be observed that a caraway “seed” is not really a seed, but consists of half the fruit; for, on careful examination, one side of it will be found to be flattened, the flattening resulting from the mutual pressure of the two half-fruits at that part; moreover the dried style exists at its summit. In the figure ([Pl. I.] fig. 19), the flattened part of the seed is next the observer. The seed has five evident longitudinal ridges, one at each corner or angle. The vittæ are dark-coloured (fig. 19 a), and placed one between each pair of these ridges; and they consist of long flattened spaces in the substance of the pericarp, with transverse markings, indicating internal cross partitions. In botanical works, the presence of five ridges, with single vittæ in the intervals, is given as a character by which the half-fruits (carpels) of the caraway are to be distinguished. But on closely inspecting the flattened surface, another ridge is seen running down its middle; so that the seed really has six ridges, one of which is smaller than the rest from the pressure of the other half. Hence the character of five ridges with single vittæ is incorrect.

The vittæ contain the volatile oil to which the fragrance and pungency of the fruit is owing, although some of the oil exists also in the cells of the kernel or albúmen, which forms the white and greater part of the seed.

The skin of a reddish apple, peeled off in the manner described for petals, exhibits beautifully the red colouring matter of different tints in adjacent cells, while the pulp displays the cell-contents, as already mentioned. The latter may also be easily examined, from their large size, in most of the softer fruits, as that of the snowberry or the cucumber.

As the ovary or fruit approaches maturity, the petals and stamens wither and fall off, the calyx often remaining, and being sometimes adherent to the ovary, at others free or unattached to it.

Seeds.—During the ripening of the fruit, the seeds contained within it are gradually becoming further developed.

The seeds themselves are covered outside by a skin or coat called the testa (testa, a shell). This is remarkable for frequently displaying various kinds of figured patterns, consisting of raised networks, ridges, little knobs or tubercles, &c. Examples of these may be found in the seeds of the poppy ([Pl. I.] fig. 27), mignonette (fig. 29), and chickweed (fig. 51).

Some seeds are winged, as it is called, i. e. furnished with an extension of the testa beyond the margin of the seed. This not unfrequently consists of aggregated fibre-cells, the spiral fibre being very distinct, as in the seeds of Eccremocar´pus scáber ([Pl. I.] fig. 52). In the seeds of another curious plant in this respect, viz. Collómia grandiflóra, the fibre-cells are separate, so as to resemble hairs, and very mucilaginous, and in the dry seed are closely pressed to its surface. If a portion of the testa of these seeds, which can be procured at the seed-shops, be cut off, laid on a slide, a cover applied, and when the object is in focus, a drop of water be added, in a short time water softens the mucilaginous walls of the cells, the power of the spiral fibres comes into play, and the cells expand so as to form a very interesting object; the cells, in their expansion, apparently writhing like so many minute worms ([Pl. I.] fig. 35).

The seed itself, which is contained within the testa or seed-coat, consists essentially of the young plant or embryo. This is composed of three parts, viz. the plúmule (plumula, a little feather), or the young stem; the rad´icle (radicula, a little root), or the young root; and one or two, rarely more, imperfectly developed or rudimentary leaves, the cotyle´dons (κοτυληδὠν, a cup).

These structures are closely packed in the seed, and are not easily recognized at first. By keeping seeds moist for a day or two until they begin to grow, or germinate as the seed-growth is called, they are readily detected, and may then be more easily found in the dry seed.

When somewhat advanced in growth, they are familiar to every one, although they may not be recognized by their names. In table “mustard and cress,” the whole consists of these organs of the two plants; the white stalk directed downwards being the radicle, the two green leaf-like lobes the cotyledons, and between the latter directed upwards is the very minute plumule, which is more easily seen when the plants have been allowed to grow larger. This structure of the seed is important to be known, because the absence or presence and the number of cotyledons afford characters, corresponding with those already mentioned in respect to the veins of the leaves and the structure of the stem, for distinguishing the great divisions of the Vegetable Kingdom. Thus, the Exogens are Dicotyledons (δις, twice), their seeds having two cotyledons; while the Endogens are Monocotyledons (μὁνος, single), having one only; and the Cryptogam´ic plants are Acotyledons (α, without), their seeds (spores) having none of these organs.

Some seeds consist entirely of the embryo, surrounded by the testa. But in many others there is also present a usually whitish, firm cellular substance, called the albúmen (albumen, white of egg).

The albumen of seeds often affords good specimens of secondary deposit, the cells being almost entirely filled with it. An example may be found in a section of vegetable ivory, of which ornaments are sometimes made; its structure resembles essentially that of the plum-stone. In other instances the cells contain secreted matters, as starch, oil, &c.; and sometimes the cotyledons also contain starch and oil. An example of the former exists in the albumen of wheat; and of the latter, in the horse-chestnut, the filbert, and mustard-seed.

The albumen and cotyledons serve to supply the embryo with nutriment until the roots have grown sufficiently to enable them to absorb it from the soil; the cotyledons also serve as temporary leaves.

The form and relative position of the radicle and cotyledons serve to distinguish certain groups of plants. This may be illustrated by the natural order Cruciferæ, or that containing the mustard, wall-flower, &c.

Thus, in one group, which may be represented by the wall-flower, the cotyledons are flat or plane ([Pl. I.] figs. 43 & 44), the radicle being applied to their edges. This is best seen in a transverse section (fig. 43). They are then called accum´bent (accumbo, to lie against); and the botanical sign is O=. In the second group, the cotyledons are plane ([Pl. I.] fig. 38), with the radicle applied to the back of one of them, as in the seed of the common shepherd’s purse (Capsel´la bur´sa pastóris) ([Pl. VII.] fig. 19). They are then termed in´cumbent (incumbo, to lie upon), and the sign is O||. While in the third group the cotyledons are folded in the middle, like the leaves of a book (Pl. I. figs. 49 & 50), and the radicle is enclosed between them, as in the white mustard (Sina´pis alba). The cotyledons are then called condu´plicate (conduplico, to fold); and their sign is O> >.

The plants above-mentioned are evidently all Dicotyledonous, or their seeds have two cotyledons; and they contain no albumen.

In the Monocotyledonous division, which may be represented by a grain of wheat ([Pl. I.] fig. 53), the single cotyledon forms a minute sheath (a), enclosing the plumule (b), the radicle (c) being here but little developed at first, the greater part of the grain consisting of the albumen (d); the grain should be softened in water before examination. In the germinated grain the cotyledon appears as a pale sheath, surrounding the convolute green leaves of the plumule; which may be best seen in a transverse section ([Pl. I.] fig. 48).

Fertilization.—A few words must now be said regarding the formation of seeds, and the action of the pollen-tubes in the process of fertilization.

In the earliest stages of growth, the young seeds, or ovules as they are called, appear as little buds, arising from the inner wall of the ovary; and the part from which they arise is called the placen´ta (placenta, a cake). In chickweed ([Pl. I.] fig. 41 c), the placenta forms a central column; and when the ovules are a little older, they are found to have separated somewhat from the placenta, but retaining a connexion by means of a little cord or stalk, termed the funic´ulus (funis, a cord). The ovules may be readily found in the ovary or young pod of a wall-flower, the placentas forming four lines, running longitudinally down the interior of the pod.

In this early condition the ovule consists of a mass of cellular tissue; and as new formations are soon added to it, it is termed in this state the núcleus. Around the nucleus are then formed two coats, an outer, called the prímine, and an inner, termed the secun´dine. These coats or membranes are open at one end, so as to leave a passage down to the apex of the nucleus; the opening is called the forámen. These structures are well seen in the ovule of the wall-flower ([Pl. I.] fig. 54), the foramen in the figure being indicated by a *; it will be noticed also that the funiculus runs down one side of the ovule, so as to terminate at the bottom or base of the nucleus. In ripe seeds, the spot at which the funiculus has been attached is mostly perceptible in the form of a scar. The slight prominence of the foramen can also often be distinguished, as in the seed of chickweed ([Pl. I.] fig. 51*); in the ripe seed the foramen is termed the mícropyle, and towards it the radicle of the embryo is always directed.

One of the cells of the nucleus near its apex then enlarges, so as to form a sac, called the embryo-sac. This is excessively thin and transparent ([Pl. I.] figs. 45 b & 47); and in it, also at the end next the foramen, one or more (in the chickweed one) smaller cells are formed from the cell-contents of the embryo-sac, which are called the embryonal vesicles ([Pl. I.] fig. 45 a).

Thus far developed, the embryo exists prior to the expansion of the flower and the discharge of the pollen. The embryo-sac is not figured in this early condition, the embryonal vesicle being then smaller than that in fig. 45 b, although occupying the same position.

When the pollen has escaped from the anthers and fallen upon the stigma, the pollen-tubes growing down the intercellular passages of the style, enter the foramen of the ovule, and so reach the apex of the nucleus, at which the embryonal vesicle contained in the embryo-sac is situated. The end of the pollen-tube then adheres to the embryonal vesicle, and such interchange of cell-contents takes place between them as effects fertilization.

The process of cell-formation in the fertilized embryonal vesicle then takes place rapidly, new cells being formed by the division of its cell-contents ([Pl. I.] fig. 45 a); and it will be noticed that the new cells are formed at the end of the embryonal vesicle, opposite to that situated at the apex of the embryo-sac. As the cell-division and formation proceed further, a mass of new cells is produced ([Pl. I.] figs. 46 c & 47), forming the rudimentary embryo; and from this, by further growth, the perfected embryo (fig. 55) results; or, to use a fashionable technical term, the simply cellular embryonic mass becomes differentiated into the radicle, cotyledons, and plumule, forming the embryo. It will be remarked that the position of the embryo in fig. 55 is the reverse of that in figs. 46 & 47, the radicle in the former being directed downwards, whilst that of the embryo in the figure of the embryo-sac (fig. 47) is directed upwards.

The embryonal structures are very difficult of detection; but it happens that in our little chickweed they are more easily dissected out than in most other plants. For this purpose, the ovules, placed on a slide and lying in water, should be picked to pieces with the mounted needles, under the simple microscope. They may be preserved in chloride of calcium or glycerine.

A clear distinction must be drawn between seeds, which result from the process of fertilization, and buds, which are formed independently of this process. Both consist essentially of embryo plants; but while the former originate from a single cell, the latter are outgrowths of a parent stem, from which their tissues are derived; and while the former propagate the species, the latter increase the individual.

The obvious use of seeds is the distribution of the species by the formation of new individuals.

In the general outline which has been given of the elements, tissues, and organs of plants, they have been examined principally as existing in the higher groups, or those of more complex structure; and to enter further upon a description of these plants would involve the consideration of variations in the form and arrangement of the organs of which they are composed. As these can mostly be investigated without the use of the microscope, we must pass to those in which the entire plant consists of little more than simple or parenchymatous cells, and in which the representatives of the flower are so inconspicuous, or are reduced to so elementary a condition, that the plants included in the Division have been termed Cryptogam´ic (κρυπτὀς, concealed, γἁμος, union—figuratively for reproductive organs) or Flowerless Plants. The reproductive organs of the Cryptogamia are usually termed the fructification, implying that they produce fruit, but not flowers.

CHAPTER V.
FERNS, OR FIL´ICES.

THE general appearance of plants belonging to the class of Ferns is so well known that it need scarcely be described, especially since the introduction of the glass plant-cases, by means of which the air can be kept so damp that ferns are now grown in the very heart of our cities. Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or fronds (frons, a leaf) as the leaf-like organs of the lower plants are called, arising in tufts from the stems, give them the elegant appearance we are called upon to admire, whenever they are met with. The brownish spots or stripes seen upon the back or under surface of the fronds, and consisting of the fructification, form also a simple character by which they may generally be distinguished; although in a few of them the fructification is placed upon a distinct stalk. The stem or rhi´zome (ῥἱξωμα, a root) of a fern is mostly situated just beneath or at the surface of the ground, and is commonly mistaken for the real root, which is buried in the earth. It is brownish outside, and covered with scurfy scales or ramen´ta (ramentum, a shaving). These scales are interesting microscopic objects, from the distinctness with which they exhibit the cellular network.

A section of the rhizome exhibits the fibro-vascular tissue arranged differently from that in the stems of either Exogens or Endogens. It forms curiously curved longitudinal plates, a very abundant component of which is the scalar´iform (scala, a ladder) duct ([Pl. II.] fig. 34). The walls of the scalariform ducts are angular, and the secondary deposit is arranged in the form of transverse bars, somewhat resembling the steps of a ladder; which structure is best seen in a transverse or slightly oblique section. The fronds are usually cleft nearly down to the main vein or midrib (fig. 9), or pinnatifíd (pinna, a feather, findo, to cleave); sometimes the segments are similarly cleft, so that the fronds are bipinnatifid. The manner in which the veins usually branch is also peculiar, each branch separating from the point of division at an acute angle with the original direction of the vein (fig. 20), so as to be forked. It is also worthy of notice, that the young frond is rolled up into a flat spiral, or is cir´cinate (circino, to go round), before it opens.

We will now examine one or two species more minutely.

Polypodiaceæ. Polypódium vulga´re ([Pl. II.] fig. 9, one-third of the natural size), a member of this family, is very common on old trunks of trees, on banks, &c. The frond is deeply pinnatifid, the segments being oblong, blunt (obtuse), scalloped (crenate) at the edges (PI. II. fig. 10), and becoming gradually shorter towards the apex of the frond.

On the back of the fronds are the little orange-coloured groups of capsules ([Pl. II.] figs. 9 & 10); these are called sor´i (σορὀς, a heap). The capsules or thécæ (θἡκη, a case), a magnified group of which is represented in fig. 11, consist each of an aggregation of cells, fixed to a stalk (fig. 12); and along the back of the capsule is a close row of thicker cells, forming an elastic ring, the an´nulus. When the seed-like bodies or spores (σπορἁ, a seed) are ripe, the annulus becomes straightened from its elastic power, and tears the capsule open, so that the spores are set free and scattered.

Aspid´ium filix mas ([Pl. II.] fig. 16, reduced to one third or fourth of the natural size), is the most common British fern. In this the fronds are pinnate, i. e. the segments (pinnæ) corresponding to those of Polypodium are cleft entirely to the main stalk, the pinnæ (fig. 17) being pinnatifid; and the segments of the pinnæ or the pinnules (fig. 18) are oblong, obtuse, and saw-edged (serrate). In all botanical works the fronds are incorrectly said to be bipinnate.

The sori are brown, kidney-shaped (fig. 18), and differ from those of Polypodium in being covered by a thin membrane (indúsium), which is fixed to the frond at the notch.

Scolopen´drium vulgáre ([Pl. II.] fig. 19), the Hart’s-tongue Fern, is common in hedges and on moist banks. Its fronds are simple or undivided, strap-shaped, heart-shaped at the base, and narrowed to a point at the apex. The sori (fig. 20) are brown, narrow, longish (linear), and transverse or slightly oblique. The indusium is cleft down the middle, so as to form a longitudinal fissure or suture.

Reproduction.—The spores of the ferns ([Pl. II.] fig. 12 a) resemble in appearance the seeds of flowering plants on a small scale. They are usually brown, covered on the surface with little tubercles or other markings, and when kept on a slide in a moist atmosphere, as over a saucer of water covered with a bell-glass, they germinate. When this takes place, one or more short, brownish, hair-like radicles emerge from one part of the surface ([Pl. II.] fig. 13), and a process containing chlorophyll issues from an adjacent spot. As growth proceeds, the latter by cell-division gives rise to a flattened two-lobed leafy cotyledon-like body or prothal´lium (πρὀ, before, θαλλὀς, leaf), with numerous rootlets springing from the base of the lobes. The prothallium is of a peculiar dull-green colour, different from that of the young frond which is subsequently formed. This arises from the absence of stomata and intercellular passages containing air; for the air in these passages of leaves and petals contributes greatly to the production of the brightness of their colours. Moreover, the cells of the prothallium resemble those of the parenchyma of a leaf, the epidermis with its wavy-margined cells being absent.

When the prothallium has attained its full development, minute scattered protrusions from its cells occur on the margin or under surface, resembling short and blunt hairs; and each of these becomes partitioned off to form a new cell, within which a number of crowded smaller cells are produced. These organs are called antherid´ia (anther, and εἶδος, resemblance); and within each of the crowded smaller cells is contained a very minute, colourless, coiled fibre, furnished with still finer filaments, called cil´ia (cilium, an eyelash); the ciliated fibres being termed spermatozo´a (σπἑρμα, seed, ξῶον, animal). At a later period, other organs are found also on the back of the prothallium. These are larger than the antheridia, and are composed of several cells, arranged around a central canal which leads to an embryo-cell situated at its base ([Pl. II.] fig. 2). These organs are the archegónia (ἁρχἠ, beginning, λὁνος, offspring). When the antheridia are ripe, they discharge the spermatozoa, which are enabled to swim about by means of their cilia in water (rain), and entering the canal, reach the embryo-cell, which thus becomes fertilized. When fertilized, the embryo-cells produce the little fronds which afterwards grow into the mature plants.

Hence the spores of ferns differ strikingly from the seeds of the higher plants in not containing the embryo radicle and cotyledons already formed, these being produced during or after germination; also in the fertilizing organs, viz. the antheridia or representatives of the anthers, and the archegonia or the representatives of the pistils, being produced from the cells of the prothallium.

The more minute of these structures are too difficult of observation and preparation for any one unaccustomed to microscopic manipulation, so that they have not been figured in detail; the figures given will, however, serve to guide the observer in their recognition.

Preservation.—The ferns may be easily preserved in the entire state, by laying them flat between sheets of coarse unsized paper, and subjecting them to moderate pressure in a screw-press; the paper should be changed, or dried before a fire every two or three days, and the pressure repeated until the specimens become dry and rigid. They may then be mounted on sheets of paper, being fastened either with thread passed round the stalk or portions of the frond with a needle, and tied in a knot behind, or with strips of paper gummed at the ends.

The minute structures may be preserved either in the dry state or in glycerine.

CHAPTER VI.
MOSSES, OR MUS´CI (MUSCUS, MOSS).

I NEED scarcely refer to the figures in [Pl. III.] to enable the reader to recognize the Mosses; every one knows them at once by their remarkably uniform general appearance, their miniature-plantlike form, their crowded little leaves, concealing the slender wiry stems, their growth in patches, and their curious urn-shaped fruits raised up on slender bristle-like stalks.

The leaves of the mosses are simple, i. e. not cut into segments, and consist of one or two layers of cells. The thinness of the leaves enables these cells to be seen very distinctly, the closely united cell-walls giving the leaves a netted or reticulated appearance (fig. 48), and the grains of chlorophyll being generally few and readily distinguished. The veins of the leaves, or the nerves as they are usually called, scarcely deserve the name; for neither they nor even the stems contain fibro-vascular tissue, but consist simply of elongate closely packed cells, and often the leaves have no nerves.

The fruit of the mosses consists of a capsule, sometimes called a sporan´gium (σπορἀ, seed, ἄγγος, vessel), usually placed at the end of a slender stalk, called the séta (seta, a bristle); but sometimes the stalk is absent or extremely short, when the capsule is said to be ses´sile (sessilis, sitting). The young capsule is covered with a thin extinguisher-like cap or calyp´tra (καλὑπτρα, a cover), which is carried up as the capsule and its stalk grow, so as to be either entirely thrown off, or to remain covering a greater or less portion of the capsule, when this attains maturity.

The calyptra is either simply mitre-shaped, or mítriform ([Pl. III.] fig. 24), or it is half-cleft, or dimid´iate (figs. 14, 36). When the capsule is ripe, the upper part usually separates at a circular horizontal line (fig. 8) as a kind of lid, which is called the oper´culum (operculum, a lid), and thus the spores are enabled to escape. The rim of the capsule, from which the operculum has separated, forms its mouth, and this often exhibits a fringe of teeth (figs. 15, 18, 31), arranged in one or more rows; sometimes the teeth are replaced by a membrane, or, again, both teeth and a membrane may be present. This mouth-fringe is the per´istome (περἰ, around, στὁμα, mouth). In many mosses, an elastic row or ring of cells is situated between the mouth of the capsule and its operculum, called the annulus (figs. 18 & 39); this, when the capsule is ripe, aids in throwing off the operculum.

It is important to become acquainted with the structure and arrangement of these parts, as they form characters by which the families and genera of mosses are distinguished.

The capsules of the mosses form very beautiful microscopic objects, especially those furnished with a toothed peristome.

Most of the mosses produce their fructification in the winter and spring.

The class of mosses is divided into two Orders, according to whether the fruit-stalk is terminal, i. e. arises from the end of the stem or its branches, or whether it is lateral, arising from the side of the stem. Those with the fruit-stalk terminal, or the end fruited ([Pl. III.] fig. 22), form the Ac´rocarpi (ἄκρα, summit, καρπὀς, fruit); while those with the fruit-stalks lateral, or the side-fruited mosses (fig. 43), constitute the Pleu´rocarpi (πλευρἀ, side). The new shoots or young branches of the stems of mosses are termed innovations.

We will now examine a few common mosses more in detail, beginning with the Acrocarpi.

Sphag´num acutifólium ([Pl. III.] fig. 4) is found in pools or bogs, growing at the margins so as to be partially immersed. In this moss, the upper branches are grouped into a head. The leaves are crowded, and overlapping or im´bricate (imbrex, a tile) on the elongate stems; they are egg-shaped (ovate) on the main stems (fig. 1), and narrower or ovate-lanceolate on the branches; they are nerveless, and finely toothed at the apex. The capsule (fig. 5) is roundish-ovate, without a peristome, and the operculum is flattened. The grouped arrangement of the upper branches renders the species of Sphagnum easily recognized. The structure of the leaves is also very peculiar and characteristic (fig. 2). The cells of which they consist are of two kinds, one (fig. 2 a) being colourless, elongate, pointed, and containing a spiral fibre; the other consisting of shorter and narrower obtuse cells, containing chlorophyll, and situated between the former. In many of the former kind of cells, little round apertures exist on the under surface, and minute animals may sometimes be found imprisoned in them.

Another species of Sphagnum, S. obtusifolium, is common, and greatly resembles the above, but has shorter and thicker stems, and rounded-ovate, very concave, and obtuse leaves.

Gymnos´tomum truncat´ulum ([Pl. III.] fig. 6) is a common little moss, found on banks and in fields and gardens.

In this there is no peristome, although, in the young condition, a membrane extends more or less over the interior of the mouth of the capsule. The stem is slender, rigid, and simple, or but little branched. The calyptra is dimidiate; the operculum is present (fig. 8), and terminates above in an oblique beak, or it is obliquely rostrate (ros´trum, a beak) as it is called. The leaves are obovate (fig. 7) or ovate with the broader part remote from the stem, and narrowed at the apex, where the nerve protrudes or is ex´current (excurro, to run out). The spores (fig. 9) are reddish brown and smooth.

Dicránum heteromal´lum ([Pl. III.] figs. 10 & 11) is probably the first moss the reader will meet with on banks and heaths in the early spring; and it will be sure to be noticed on account of the bright green colour of the patches and the beautiful orange-brown capsules.

In this moss the capsule is nodding (cer´nuous) ([Pl. III.] fig. 15), and has a single peristome, consisting of sixteen equidistant teeth, each being deeply cut or cleft longitudinally (fig. 15 a), so that there are thirty-two teeth altogether; and these are marked with internal cross-bars, or transverse ridges. The calyptra is dimidiate (fig. 14); and the lid is furnished with a long oblique beak (fig. 15 b). The leaves are crowded, strongly nerved (fig. 12), lanceolate at the base, and very narrow towards the apex, which is toothed; they are, moreover, curved, and bent towards one side, or sécund.

Tor´tula murális ([Pl. III.] fig. 16) may be found on the top of almost every wall and on waste ground.

In this moss the peristome is single (fig. 18), consisting of thirty-two spirally twisted teeth, arranged in pairs. They are narrow and slender, and each is composed of two longitudinal portions (fig. 18 a), one of which is pale yellow, the other reddish brown, like the capsule, and both are fringed and covered with very minute papillæ. The capsule (fig. 18) is oblong, the ring or annulus remaining for some time. The lid is conical (fig. 19), with a longish somewhat oblique beak, and the calyptra is dimidiate (fig. 20). The stems are very short; the leaves (fig. 17) are oblong, obtuse; the nerve strong, and projecting as a colourless spirally striated bristle. The bristles often give the patches of the moss a hoary appearance on wall-tops. The margins of the leaves are folded back or recurved, giving them a peculiar thickened appearance.

The largest of our mosses are contained in the next genus, viz. Polyt´richum, some of them having the stems from 2 to 4 inches, or even more, in height; they are common on heaths and in woods.

Polyt´richum pilif´erum ([Pl. III.] fig. 22) is very common on open dry heaths. This moss has simple stems, with the leaves crowded on the lower part of those which are fertile or fruit-bearing. The fruit-stalk is terminal (acrocarpous); the capsule ovate, 4-sided or quadrangular, with a knob or struma (strúma, a swelling) at the base, the lid having a short beak. The calyptra (fig. 24) is half-cleft (dimidiate) and very hairy. The peristome is single, and consists of sixty-four teeth. The leaves (fig. 23) are lanceolate, nearly upright, the margins folded inwards or inflexed; and they end abruptly in a saw-edged or serrated hair-like point.

Poly´trichum commúne, which is also very common, is larger than the last species, and may easily be distinguished by the curved and serrate leaves, which have no bristle-point.

In the early spring, patches of both these mosses may be found, in which the stems are terminated by little rosettes (figs. 25 & 26); these will be referred to presently.

Keeping still to the end-fruited or Acrocarpous mosses, we have next to mention Funa´ria hygromet´rica ([Pl. III.] figs. 28, 29), which is readily distinguished from most other mosses by the pale apple-green colour which it possesses before the capsule ripens. It is extremely common on walls and waste ground.

The capsule of this moss (fig. 31) differs from those of the preceding mosses in the peristome being double (fig. 42), or composed of an outer and an inner row of teeth. The outer row consists of sixteen oblique reddish teeth, which are marked with transverse bars or trabe´culæ (trabecula, a little beam), and their points are connected by a net-like thin plate. The inner row contains also sixteen teeth, arising from the division of the membrane lining the capsule; these are yellowish, thin, and placed opposite the outer teeth. The capsule itself is pear-shaped or pyriform, orange-red when ripe, curved, and with the mouth oblique. The calyptra (fig. 36) is half-cleft, and expanded as if blown out below. The lid (fig. 31 a) is convex and obtuse; and the annulus (fig. 39) is large and easily separable. The fruit-stalks are curved near the top. The leaves (fig. 30) are ovate, concave, entire, with a nerve reaching the apex, which is acute and prolonged into a little point, or apic´ulate. The spores (fig. 38) are small and reddish brown. The specific name (hygromet´rica) of this moss expresses its hygrometric property; for if either the recent and moist moss be dried or the dry moss wetted, the fruit-stalk gradually twists in opposite directions in the two cases.

The last of the Acrocarpous mosses which we shall notice, Bry´um capil´lare ([Pl. III.] fig. 49), is tolerably common on trunks of trees, on the ground, and sometimes on walls.

The capsule of this moss has a double peristome or mouth-fringe; the outer consisting of sixteen reddish-brown, equidistant, transversely striped teeth; the inner composed of sixteen thin keeled teeth, more or less split down the middle, and with two or three intermediate cilia. The capsule is nodding, smooth, oblong, pear-shaped, slightly narrowed below the mouth; the lid being somewhat convex, and furnished with a short slender beak. The calyptra is dimidiate. The leaves (fig. 50) are obovate, the nerve extending beyond the point, rendering them bristle-pointed. The seeds (fig. 49 a) are small and green.

This moss serves to illustrate a great difficulty, which will often occur to the student, in determining whether a moss is end-fruited or side-fruited. For in this, as in many other end-fruited mosses, a little side-shoot or young branch (innovation) grows from the main stem immediately below the leaves surrounding the base of the fruit-stalk, so that the fruit-stalk appears to arise from the side of the stem. The only method of overcoming the difficulty is to examine carefully the comparative size and thickness of the stem and the shoot, and to determine which is the weaker and so the newer. The leaves surrounding the base of the fruit-stalk, which are mostly somewhat different in structure from the stem-leaves, are called the perichæ´tial (περἰ, around, χαἱτη, bristle) leaves.

From among the side-fruited or Pleurocarpous mosses we shall select one only, Hyp´num rutab´ulum ([Pl. III.] fig. 43), which is common on the trunks of trees and on banks.

In this moss, the nodding unequal curved capsule (fig. 45) has a double peristome, resembling that of Bryum (fig. 47). The calyptra is half-cleft, and the lid conical and shortly beaked. The stem is reclining or procumbent, and the pale green imbricated leaves (fig. 41) are ovate and pointed, faintly saw-edged, the nerve becoming indistinct at about the middle. It will be noticed that the cells of the leaf (fig. 48) have the prosenchymatous form, or are elongate with pointed ends; and that the fruit-stalk (fig. 45) is rough with little grains.

Fructification.—The fruit-producing organs of the mosses are of two kinds, comparable to those of the flowering plants, but with their names changed, as in the case of the ferns; the representatives of the anther being called antheridia, and those of the pistil archegonia. The antheridia may be best examined in Polytrichum piliferum or commune, the patches of stems with red rosette-like heads (figs. 25, 26) being readily found in the spring on open heaths. The coloured leaves forming these heads differ in form from those of the stem, being broader and very sharp-pointed, and have received the distinctive name of perigónial (περἰ, around, γνος, offspring) leaves. In the centre of these leaves, which must be separated with mounted needles in a drop of water, the antheridia (fig. 27), forming oblong cellular green sacs, will be seen; and intermingled with them will be found some slender pale filaments, composed of mostly two rows of cells, which are the paraph´yses (παρἁφυσις, a side growth). If the antheridia are quite ripe, they swell somewhat in the water, and from the free or unattached end a very delicate, colourless, cellular mass gradually escapes. If the antheridia are not quite ripe, the mass must be liberated by dissection.

On carefully examining this mass under a high power, it will be seen to consist of very delicate rounded cells (fig. 3 a), each containing a coiled filament, revolving more or less rapidly. After a time, these filaments (fig. 3 b) escape, so that they may be examined more minutely. They are excessively delicate, and are best seen when dried on the slide. Each consists of a very slender curved filament, with a still finer filament, or cilium, arising from it on each side. These are the spermatozóa or spermatozóids (στἑρμα, seed, ξῶον, animal, εἶδος, resemblance).

In Funária the antheridia (fig. 37) may also be found, by careful examination, in the little green heads terminating some of the stems (fig. 41, of the natural size). In this moss, the paraphyses are inflated at the summit into little knobs, or they are capitate (fig. 37). The pistil-like organs of mosses, or the archegónia, from which the capsule is formed, must be looked for in the winter or early spring. They occur in the parts of the stems from which the fruit-stalk subsequently arises, and are surrounded by perichætial leaves, so as to resemble in general aspect the antheridial heads. They are readily found in Tortula and Funaria, which are always at hand.

The archegonia ([Pl. III.] fig. 21) differ in form from the antheridia, being flask-shaped, with a neck and a dilated base. The neck contains a slender canal, and within the base is a special embryonal cell, from which the capsule is subsequently formed. The spermatozoa of the antheridia pass down the canals of the archegonia, and fertilize the embryonal cells; but one archegonium only comes to maturity in each head, the others ceasing to grow, and withering, in which condition they are found at the base of the fruit-stalk when the capsule is fully formed. The embryonal cell grows by subdivision, so as to form a stalk-like body, which as it rises extends the archegonium upwards until it splits across near the base. Thus the archegonium becomes split horizontally into two parts, the upper and longer of which forms the calyptra, whilst the lower remains as a very short tube or sheath (vagi´nula) surrounding the base of the fruit-stalk. The cellular stalk-like body then swells at the summit, the swollen portion gradually becoming developed into the capsule, by resolving itself into an outer wall lined inside with a coat forming the outer row of teeth at the top, and within this a thinner membrane or spore-sac, the cleft upper margin of which forms the inner teeth; and within this are contained the spores. The mass of cells within the spore-sac remains, forming a central column, called the columella.

These stages of growth may be readily traced in Funaria. In Plate III., fig. 40 represents two fertilized archegonia of the natural size, surrounded by the perichætial leaves; fig. 33 is a still more advanced archegone. In fig. 34 the calyptra has separated from the vaginule, and contains the stalk-like body, which is represented alone in fig. 32, the dark summit indicating the commencing formation of the capsule. Fig. 35 represents the young capsule, in which all the parts are more advanced in growth.

When the seeds of mosses germinate, they produce at first a green Conferva-like filament, which branches at one end, the cells containing green endochrome, while brownish little roots are given off from the other end. The young leafy buds or young stems arise from these confervoid filaments.

Examination.—In the examination of the mosses, the capsules should be viewed as opake objects while fixed in the forceps; and to discover the minute structure of the teeth of the peristome, a capsule should be wetted with spirit, then immersed in water, slit up with fine scissors, and spread out with the mounted needles, so as to form a transparent object. In this way, the curious structure of the teeth becomes very distinct.

It must be noticed that, in the mosses, the antheridia and the archegonia usually occur in separate flower-like heads; or the mosses are either monœcious or diœcious (p. 38).

Preservation.—The mosses may be dried under pressure, and preserved entire in the same manner as the ferns or the flowering plants. If simply dried without pressure, their structure can be readily made out at any future time, by immersing them in water, or by keeping them for a few hours in a moist atmosphere. The minute structures of mosses may be mounted in solution of chloride of calcium, or in glycerine; they keep extremely well without closing the cells.

CHAPTER VII.
ALGÆ (ALGA, SEA-WEED).

THE plants belonging to the Class Algæ grow in water, either in that of the sea or in fresh water; a few of them, however, being found on damp earth, damp walls, &c. The marine Algæ are commonly known as sea-weeds; but the fresh-water Algæ generally receive but little popular notice, forming, as they do, slimy masses or strata, of a green or brownish, sometimes red, colour.

Algæ are of simple structure, consisting entirely of cells; in some these are single, in others, united end to end, to form threads or filaments, or grouped into a leaf-like expansion, or collected few together into a little spherical group or a flat plate. They possess none of the fibres, vessels, or ducts of the higher plants, although some long and slender cells, existing in the stalks of the fronds of the larger kinds, bear considerable resemblance to woody fibre. They exhibit no distinction of stem and leaf, but consist of fronds representing the stem and leaf combined and undistinguishable. And the term frond must be understood to signify the separate parts arising from the point of attachment when they are fixed; and in the case of those which are unattached or free, the entire plant is called a frond.

The Algæ are divided into three Orders, viz. the Fucoid´eæ or olive-coloured Algæ, the Florid´eæ or red, and the Confervoid´eæ or green Algæ.

Fucoid´eæ, Fucoid Algæ, or Melanospor´eæ (μἑλας, black or dark). The plants composing this order form our largest sea-weeds, and are found everywhere in the sea and on the sea-shore. They are of an olive-green or olive-brown colour, and usually become darker on drying.

Fúcus vesiculósus, with its parallel-sided or linear olive-brown fronds, is known to every one as the seaweed which is hung up to act as a weather-glass. The fronds have a central stout vein, or midrib, and scattered air-bladders, mostly in pairs.

The fructification consists of yellowish oval enlargements of the ends of the fronds, called the receptacles (fig. 16); but these are somewhat variable in form, being often angular or truncate. On holding one of the receptacles to the light, it will appear to contain a number of little grains imbedded in its substance, slightly projecting above the surface, and in the centre of each is a minute dot or pore. These grains are the capsules, or conceptacles, and contain the spores. The substance of the receptacles is composed of a beautiful network of colourless, jointed, cellular fibres (figs. 17 a and 18 a), the meshes of which are filled with a transparent gelatinous substance; but immediately around the conceptacles the cells are shorter and more closely packed. The spores (fig. 19) are arranged in the conceptacles in a radiate manner; they are brown, and surrounded by a colourless sac, called the perispore (περἰ, around, σπορἀ, seed); and between them are numerous slender, colourless, jointed filaments, the paraph´yses. The spores are not, however, truly single spores, for they ultimately divide into eight segments or sporules, each of which is capable of producing a new plant.

In the conceptacles of some fronds of Fucus no spores will be found, the conceptacles (fig. 18) being filled with elegantly branched colourless filaments (fig. 20), the ends of many of them being distended into little yellowish sacs; these are the antherid´ia. The antheridia contain large numbers of exceedingly minute spermatozoa, furnished with two cilia, and very similar to those existing in the antheridia of the mosses; these, escaping through the pore of the conceptacle, fertilize the spores.

The figure (20) in the plate was drawn from a conceptacle of Fucus serrátus, another common species, differing from F. vesiculosus in having the margins of the frond serrate; the antheridia of the two species do not, however, differ in any important respect. To examine the conceptacles of Fucus and their contents, the receptacles should be soaked in water, if not fresh, and thin sections made with a sharp knife. They form very beautiful objects, and may be preserved in chloride of calcium or glycerine.

Florid´eæ, or Rhodosper´meæ (ῥὁδον, rose, σπἑρμα, seed).—The second Order of Algæ, forming the Florideæ (flos, a flower), comprises the red sea-weeds; a few of them are purple, or greenish-red; so that by the colour alone they may be readily distinguished from the Fucoids, and from nearly all those of the next Order, the Confer´voids. A few of them are leaflike, or possess flat fronds; but most of them consist of finely divided or feathery fronds. They are often found upon the sea-shore of a dirty white colour, the colouring matter having been decomposed or washed out by rain.

We shall consider a few of the genera and species under the heads of the families to which they belong.

Corallina´ceæ, the Corallines, or calcareous Algæ.—In this family we have the beautiful Corallína officinális ([Pl. IV.] fig. 28), the common Coralline, which is very abundant on the sea-shore, attached to larger sea-weeds, shells, and rocks. It is hard and chalky, from the presence of a large proportion of carbonate of lime in its minute cells. The fronds are composed of jointed and branched filaments. The fructification (figs. 29 and 30) consists of ovate cellular capsules, or ceramid´ia (κερἁμιον, earthen vessel), placed mostly at the ends of pinnate stalks, and containing a tuft of somewhat club-shaped jointed spores, springing from the base of the capsules (fig. 30). When ripe, the spores escape from a pore or hole in the end of the capsules. The spores are 4-jointed, and hence are called tet´raspores (τἑτρα, four).

To observe these spores, the capsules must be soaked in strong vinegar for some hours, and then washed with water, to dissolve the calcareous matter.

Jánia rúbens ([Pl. IV.] figs. 7 and 8) is another common and very elegant little coralline, and is of a pale red colour. It differs from the last in the branches being dichot´omous (δἱχα, in two, τομὀς, cutting) or forked, instead of pinnate. The capsules, or ceramidia, have also two short horn-like branchlets, placed one on each side, near the end.

The genus Melobésia has the frond crustaceous, i. e. forming a hard crust or layer. M. polymor´pha ([Pl. IV.] figs. 5 and 6) is common on shells, stones, &c. The capsules (ceramidia) here form little blunt cones, scattered over the crusts, and containing the tufted tetraspores, as in Corallina.

Lithocys´tis Allman´ni (fig. 9) is very minute, and not uncommon upon sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous layer of cells, closely investing the body to which it is attached; its fructification is unknown.

Leaving the family of crustaceous Florideæ, we shall now pass to those of softer consistence, although all the marine Algæ contain a considerable quantity of calcareous matter.

Rhodomela´ceæ.—In this family we have the large genus Polysiphónia, in which the frond ([Pl. IV.] figs. 25 and 26) is filamentous, the filaments being apparently jointed and longitudinally striated. The filaments are composed of rings of cells (fig. 27), arranged end to end, and containing dark endochrome. The ends of the colourless cell-walls separating the endochromes of the cells of adjacent rings produce the jointed appearance; while the striated appearance is caused by the dark cells being elongate and the cell-walls thick, so as to form white interspaces.

The fructification consists of capsules (ceramidia), attached to the sides of the branches, containing pear-shaped spores, with tetraspores imbedded in swollen branches of separate plants.

Polysiphónia fastigiáta (fig. 25, a small piece) is common, attached to the fronds of Fucus. Its filaments are rigid, bristle-like, of the same breadth throughout, forked, and forming globular brown or yellowish tufts, from 2 to 4 inches long. The joints are broader than long, each with 16-18 of the dark cells. In the centre of the branches of this sea-weed is a row of curious objects (fig. 26 a), consisting of a dark-coloured body surrounded with irregular spiny marginal processes, and with a colourless short process above and below. These require further investigation.

P. nigres´cens is also common among masses of seaweeds. Its filaments are brown, pinnate, the branches awl-shaped, and the joints about as long as broad.

Dásya coccin´ea ([Pl. IV.] figs. 1 and 2, representing small portions of a filament) is a very common filamentous red sea-weed of the same family. The filaments are 6-8 inches long, and bipinnate,—the larger ones somewhat resembling those of Polysiphonia, in being composed of parallel longitudinal cells, arranged round the centre, but containing also smaller intermediate cells; while the smallest branches (fig. 2), which arise in tufts, consist of a single row of cells, little longer than broad. The fruit consists of ovate capsules (ceramidia), placed at the base of the branches, and containing a round mass of spores. There is also another kind of fructification, occurring on distinct plants; this is formed of one or two rows of tetraspores, immersed in pod-like capsules, called stichid´ia (στἱχος, row).

Delesseria´ceæ.—In this family, the typical or most highly developed genus of which, Delesséria, has beautiful leaf-like rose-red fronds, we shall examine the common Plocámium coccin´eum ([Pl. IV.] figs. 23 and 22). This is of a fine red colour; the fronds are from 2 to 12 inches long, and consist of numerous branched and bushy filaments. These are compressed, with the branchlets arranged in alternate rows on the two margins of the stem. The end branchlets are acute and pectinate (pecten, a comb), or arranged like the teeth of a comb. The cells of which the filaments consist are small and angular, giving the surface the appearance of being elegantly netted under a high power. The fruit (fig. 22) consists of globular capsules, called coccid´ia (κὁκκος, a berry), placed in the axils or forks at which two branches separate, and containing a mass of angular spores. There are also tetraspore-pods (stichidia), as in Dasya; and tetraspores (fig. 24) in little leaf-like altered branches (fig. 21), called spor´ophylles (σπορἁ, seed, φὑλλον, leaf), and antheridia are present.

Rhodymenia´ceæ.—In this family we have Hyp´nea purpuras´cens ([Pl. IV.] fig. 35). The filamentous pale purple frond of this sea-weed is from 6 inches to a foot or more in length, the branches being alternate and spreading. The fructification consists of capsules or coccidia (fig. 32), immersed in the branches, and containing the spores (fig. 34). Tetraspores also occur in the cells of the surface of the filaments.

Ceramia´ceæ.—This is the last family to be noticed. Cerámium nodósum ([Pl. IV.] figs. 10 and 11), which belongs to it, is a most delicate and elegant filamentous sea-weed, commonly found attached to other sea-weeds. The filaments are hair-like or capillary, irregularly dichot´omous; they consist of colourless cells, 3 or 4 times as long as broad, and with thick walls. The junctions of the cells are swollen (fig. 11), and covered with very minute dark red cells, giving them a knotty and jointed appearance to the naked eye or under a low power. The globular capsules, or favel’læ (favus, a honeycomb), containing the numerous spores, are situated at the ends of the branchlets, and the tetraspores (fig. 11) in twos or threes on the outer margins of them.